INHIBITING GERANYLGERANYLATION BLOCKS GROWTH AND PROMOTES APOPTOSIS IN PULMONARY VASCULAR SMOOTH-MUSCLE CELLS

The activity of small GTP-binding proteins is regulated by a critical step in posttranslational processing, namely, the addition of isopreno id lipids farnesyl and geranylgeranyl, mediated by the enzymes farnesy ltransferase (FTase) and geranylgeranyltransferase I (GGTase I), respe ctively. We have developed compounds that inhibit these enzymes specif ically and in this study sought to determine their effects on smooth m uscle cells (SMC) from the pulmonary microvasculature. We found that t he GGTase I inhibitor GGTI-298 suppressed protein geranylgeranylation and blocked serum-dependent growth as measured by thymidine uptake and cell counts. In the absence of serum, however, GGTI-298 induced apopt osis in these cells as measured by both DNA staining and flow cytometr y. The FTase inhibitor FTI-277 selectively inhibited protein farnesyla tion but had a minor effect on growth and no effect on apoptosis. To f urther investigate the role of geranylgeranylated proteins in apoptosi s, we added the cholesterol synthesis inhibitor lovastatin, which inhi bits the biosynthesis of farnesyl and geranylgeranyl pyrophosphates. T his also induced apoptosis, but when geranylgeraniol was added to repl enish cellular pools of geranylgeranyl pyrophosphate, apoptosis was re duced to baseline. In contrast, farnesol achieved only partial rescue of the cells. These results imply that geranylgeranylated proteins are required for growth and protect SMC against apoptosis. GGTase I inhib itors may be useful in preventing hyperplastic remodeling and may have the potential to induce the apoptotic regression of established vascu lar lesions.